How to Know the Birds by Ted Floyd

Author:Ted Floyd
Language: eng
Format: epub
Publisher: National Geographic Society
Published: 2019-03-11T16:00:00+00:00

104 POPULATION DYNAMICS: FIND YOUR OWN NICHE

Rock Wren

Salpinctes obsoletus

THE SPECIES IS well named: Rock Wrens live among rocks. You can find them bopping around outcroppings above the timberline; they flourish amid talus pilings in the foothills; they abound in the butte-and-mesa country far from the mountains; and they make their homes on volcanic islands off the California coast. You won’t find Rock Wrens in the company of Marsh Wrens and Sedge Wrens, and you tend not to find them around Cactus Wrens and House Wrens, either. But what about Canyon Wrens?

To be sure, there is broad overlap between these two wren species of the American West. Rock and Canyon wrens often sing within earshot of one another, and a morning spent in suitable habitat may yield tallies in the low double digits of both. Nevertheless, they differ in various ecological details. The Rock Wren, with its gray-brown plumage, matches the granitic scree and outcroppings that proliferate in the region. The Canyon Wren is more likely to be seen scurrying mouselike up and down canyon walls, where its orangebrown plumage conceals it against the backdrop of sandstone-dominated substrates. Other differences in morphology, diet, and migratory strategy predispose the two wrens to further niche differentiation.

We understand the idea of the niche in human affairs, and a similar concept applies to ecology. A key element of niche differentiation, often referred to in the literature as niche partitioning, is competition and, in particular, the avoidance thereof. Competition-mediated niche differentiation may, in turn, be a major agent of evolutionary change. In this view, common ancestors of today’s Rock and Canyon wrens could have been driven by competition to diverge into the distinct species we see today.

Despite the role of competition in powering evolution, it may be relatively insignificant in contemporary ecological time. Remember, niche differentiation tends to promote the avoidance of competition. Nevertheless, the world is not overrun with Rock Wrens. If competition for food, say, or nests (§99) isn’t limiting Rock Wren populations, then what is?

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